1. Field of the Invention
The invention relates generally to control systems for controlling cyclical motion. More specifically, the invention relates to a control system for controlling straight-line shears for cutting gobs from a stream of molten glass in a glassware forming machine at a predetermined synchronized rate.
2. Description of the Prior Art
The use of shears for cutting gobs from a stream of molten glass falling from a feeder of a glassware forming machine is well-known in the prior art. Such shears are generally of either the arcuate or straight-line type, each type having a cyclically oscillating single pair of opposed blades associated with a single stream of molten glass. The blades of arcuate type shears are mounted at the ends of a pair of pivoted arms whereby the blades travel along an arc during their cycle. The straight-line type of shears are designed so the blades travel along a straight line during their cycle. An example of a prior art double gob shear assembly of the straight-line type is shown in U.S. Pat. No. 4,174,647, dated Nov. 20, 1979 and assigned to the assignee of the present invention.
Prior art shears of either the arcuate or straight-line type are generally activated by pneumatic (or fluidic) means interconnected in a complex arrangement of various valves and cylinders. The control and synchronization of prior art shears is also generally achieved by pneumatic (or fluidic) control means. An example, of a pneumatic control arrangement is presented in the aforementioned U.S. patent.
Pneumatic control systems have a well-known susceptibility to several parameters such as air temperature and humidity, mechanical friction, mechanical wear, etc. As a result, the actual point in time at which the shears effect a cut of the molten stream into a gob is variable and a function of these parameters. Moreover, the numerous pneumatic valves used in these systems have slow response times which affect the motion of the shears so that the valves must be energized in anticipation of the desired shear motion. In view of these inherent characteristics of pneumatic control systems precise synchronization of the shears has been difficult to achieve and maintain for any length of time. Furthermore, the speed of the shears has been limited because of the inability of pneumatic control systems to react faster then at a predetermined rate.
It is an object of the present invention to provide a control system for cyclical motion. Another object of this invention is to provide a shear control system. A still further object of this invention is to provide a control system for synchronizing shear operation within a predetermined range regardless of changes in ambient parameters such as air temperature and humidity and mechanical friction and wear.